This invention relates to an economical and nonpolluting method of removing residual vinyl chloride from aqueous polyvinyl chloride dispersions.
The preparation of homopolymers, copolymers, and graft polymers of vinyl chloride by suspension or emulsion polymerization is well-known. See, for example, monograph by Kainer, "Polyvinyl-chlorid and Vinylchlorid-Mischpolymerisate", Springer publishers, Berlin/Heidelberg/New York, 1965, pp. 12 et seq. and 111 et seq.
Such polymerizations are customarily conducted to a conversion of only about 80-95% because at higher conversions the polymerization rate decreases greatly. Also the product quality can be disadvantageously affected. Consequently the unreacted monomeric vinyl chloride must be removed from the charge.
The residual vinyl chloride is predominantly either dissolved in the polymer or bound by adsorption to the surface of the polymer, especially in pores. Only a minor proportion is dissolved in the water, because of its lack of solubility. The pressure in the gas space, depending on temperature and conversion, is several atmospheres toward the end of the polymerization in the absence of inert gases, but ranges below the saturation pressure of vinyl chloride.
Subsequently to the polymerization, the dispersion is customarily expanded into a closed system to remove the unreacted vinyl chloride and the expanded vinyl chloride is recovered after separating entrained water therefrom in accordance with the pressure, in the absence of air by conventional methods (by compression). The degasified dispersion, which still contains up to 2% vinyl chloride, based on the polyvinyl chloride is subjected to a thermal processing step to obtain dry polyvinyl chloride powder. Optionally, a mechanical preliminary dewatering stage is interposed. In this thermal processing step the residual monomer still present in the dispersion is emitted to a large extent into the atmosphere in the dryer exhaust air, depending on the type of drying process employed, and thus contributes pollution.
Moreover, depending on the selected drying method and on the molecular weight and porosity of polymer, there still remains up to about 1,000 p.p.m of residual monomer in the polyvinyl chloride.
This high proportion of residual monomer is undesirable for various reasons. Dependent on the processing conditions to which the polyvinyl chloride is subjected, a more or less large proportion of the monomer is liberated during the processing operation and is again discharged into the atmosphere. Also problems of occupational hazards suffered by the personnel arise during processing and, under certain circumstances, the necessity of explosion protection exists.
A portion of the residual monomer content still remains in the polyvinyl chloride even after the processing operation, limiting the utilization of this product, particularly in the fields of grocery and beverage packaging. For example, in the United States, in Sweden, and in Holland, regulations limit the vinyl chloride content the packaging materials coming into contact with foodstuffs to a maximum of 10 p.p.m. residual.
The above factors create the problem of removing substantially all unreacted, monomeric vinyl chloride economically from aqueous polyvinyl chloride suspensions, prior to the drying step, so that the vinyl chloride is not emitted into the atmosphere but instead is recovered and reused, and that after the drying step the residual monomer in the polyvinyl chloride is insufficient to have a deleterious effect either during processing or to preclude use for foodstuff packaging.
Although it is known from Fed. Rep. of German Published Application F 11 325 to separate the monomer under the effect of steam from suspensions, also those of polyvinyl chloride, moderately elevated temperatures are expressly recommended for carrying out these steps. In the indicated example, the monomer removal from an aqueous polyvinyl chloride suspension with a residual vinyl chloride content of 6-7% takes place under a pressure of about 100 torr (mm. Hg), corresponding to a temperature of about 52.degree. C. The description of the device utilized in this connection indicates a residence time is in the order of one minute. Accordingly, that Patent Application teaches the treatment of polyvinyl chloride dispersions with steam at moderately raised temperatures (52.degree. C.) and residence times of at most one minute. As will be demonstrated in the examples set forth below, the residual monomer content can be lowered, at best, to several thousand p.p.m. under such conditions.
German Pat. No. 1,248,943 discloses a process for the discontinuous removal of volatile impurities from aqueous polymer dispersions wherein steam is conducted through the boiling polymer dispersion in a specially constructed apparatus, making it possible to effect the process even with strongly foaming emulsions. As suitable operating temperatures, the patent mentions, in general and without correlation to the type of polymer, 50.degree.-100.degree. C. (column 4, line 52). In the example, the reduction of the residual monomer content is described merely in connection with the aqueous dispersion of a copolymer of acrylic acid ester and styrene within 3 hours at 69.degree. C., from 1.65% to 0.018%, based on the solids content. The amount of steam employed is about 1 kg. of steam per 1 kg. of solid matter or, expressed another way, about 0.0056 kg. of steam per kg. of solids and per minute. German Pat. No. 1,248,943 contains no teaching of how to free polyvinyl chloride dispersions of monomers by the use of comparatively high temperatures and high steam rates.
A process has now been surprisingly found for the removal of unreacted vinyl chloride from aqueous polyvinyl chloride dispersions to residual monomer contents of below 50 p.p.m. by the action of steam alone at comparatively high tempertures and high steam rates.